Constant resistance amplifier circuit



Dec. l5, 1936.

R. A. BRADEN CONSTANT RESISTANCEAMPLIFIER CIRGUI'T Filed Oct." 5, 1929 3 Sheets-Sheet I L mumuuuum INVENTOR ATTORNEY Dec. 15, 1936. R. A. BRADEN 2,064,146

CONSTANT RESISTANCE MPLIFIER'CIRCUIT Filed oct. s, 1929 s sheets-sheet 2 @ya y I 25g-5 RENE A. BRADEN ATTORN EY Dec. 15, 1,936. R A, RADEN 2,064,146 l.

CONSTANT RESISTANCE AMPLIFIER CIRCUIT INVENTOR RENE A. BRADEN ATTORN EY Patented Dec. l5, 1936 n; l

UNITED STATES PATENT OFFICE CONSTANT RESISTANCE AMPLIFIERl CIRCUIT Rene A. Braden, New York, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application October 3, 1929, Serial No. 396,956

26 Claims. (Cl. 178-44) My present invention relates to amplifiers, and, as the frequency is varied from the lower limitmore particularly, to amplifying tuned circuits ing frequency to the upper frequency limit. For' possessing either a, constant effective resistance, convenience in explanation, a tuned circuit which or an effective resistance which is inversely pro` includes'such means for adding a variable resistportional to frequency. ance will be called a constant resi-stance tuned 5 In band-pass tuned radio receivers employing circuit, it being understood that the naine in-A a pair of coupled tuned circuits in each stage, cludes circuits adjusted to have highest 'resiste it has been found necessary to insert resistance ance at the lowest frequency to which the cirin the circuits to make the received band sufcuit may be tuned, as well as -circuits adjusted ciently uniform in amplification; that is to say, to have equal resistance at the highest and low- 10I to make the amplifier amplify uniformly all band est frequencies to which the circuit may `be frequencies within the accepted band. This is tuned. necessary when the receiver is tuned to low Another impo-rtant object of the invention is wave frequencies, but is generally not necessary to provide a multi-stage radio frequency amwhen the receiver is tuned to high wave freplier which includes at least one tuned circuit 15 quencies for two essential reasons. in each pair of coupled input and output cir-W In the first place, the resistances of the coils cuits, the tuned circuit possessing a constant in the tuned circuits are low at low wave freresistance (according to the definition above) quencies, but increase rapidly with increasing over apredetermined range of frequencies whereg wave frequencies. In the second place, conby the sharpness of tuning is substantially thez siderably less total resistance is required in the same at all frequencies within the predetermined tuned circuits at high wave frequencies than at range. low wave frequencies, if the band-pass circuits Another object of the inventionis to provide are such that a constant band width is aca band-pass amplifier including a series or chain g5 cepted. It is desirable, therefore, to be able to of coupled circuits, at least two of which circuits 2'5 add resistance in such a form that its magniare tuned, the tuned circuits possessing constant tude is inversely proportional at any frequency resistance (as defined above) over a predeter-V to the frequency impressed upon the tuned cirmined range of frequencies whereby the band# cuits, so that the resistance of the circuit at pass amplifier amplies uniformly all frequenciesv each iIlCreaSng freqllelly can be made '60 dewithin the accepted band, each tuned circuit con- 30 Grease aS 1711 frequency impressed 1111011 the tuned sisting of an inductance, a variable capacity, and

circuits increases. a series impedance including a resistanceand Again, in receivers employing a single tuned Capacity in parallel. n Circuit in each Stage. it iS 1 00111111011 Chalalel- Other objects of the invention are toimprove :x5 istie 0f Such receivers to time too sharply at generally the simplicity and emciency of band- .35.: 10W WaVe freqllences al@ '000 b'l'odly at high' pass amplifiers, and to provide a tuned circuit W'aVe frequencles- In thls Case l? 1S helpful t0 which is reliable in operation, possesses constant add rsistan to the tuned mrcmts at 10W fre* resistance over a predetermined range of frequncles to make them tune less sharply but quencies, and is economical to construct.

40 reslsta'nce should not be added at hlgh fre' The novel features which I believe to be char- 40@ quencles 1. acteristic of my invention are set forth in partants r;iinteresaria? a ems, e frequencies and. 10W resistance at high fre itself, however, as to both its organization and method of operation will best be understood by h t l ffe 45 geesiuvlelrcte vlabee Slfgnllsa reference to the following description taken in 45*- same, regardless of the impressed' frequency. nection, with the ,drawings in which I have Accordingly, it is one of the main Objects of indicated dlagrarnmatically several circuit organ-- my present invention to employ n a tuning cir, izations whereby my invention may be carried,

cuit means for adding to the resistance of the into effect- 507 tuned circuit an additional resistance which shall In the d'aWmgS, vary inversely with the impressed frequency, Fg- 1 ShOWS 2 tuned CIClllt GmbOdymg 011ev whereby the total circuit resistance can be made fOrm 0f my invention, n to either remain constant at all frequencies be- Fig. 1A shows the electrical equivalent of the* tween two predetermined' limits, or to decrease circuit in Fig. 1, n, 55, f

Fig. '7 is a graphic showing of the operation of the invention as shown in Fig. 2, l

Fig. 8 discloses a band-pass amplifier embodying the embodiment shown in Fig. 1, I

Fig. 9 discloses a tuned `radio frequency amplier incorporating the modication shown in Fig. 2,

Fig. 10 is a graphic representation of the operatic-n of the circuit shown in Fig. 9.

Referring to the accompanying drawings in which like characters of reference indicate the same parts in the different views, Fig. 1 shows a tuned circuit embodying the principle of my invention. This tuned circuit includes an inductance coil I which is electrically associated with the variable capacity 2, the two elements cooperating to tune the circuitl to` any desired frequency, there being inserted between the inductance and capacity a resistance R0 in shunt with a capacity Co.

The circuit shown in Figure 1 is most readily explained by reference to the graphic representation in Figure 3, the latter disclosing the equivalent series impedance of the parallel resistance and capacity circuit. The impedance between the points a and b in Figure 1 is (Where w=21rf) Y The circuit consisting of Ru and Co in parallel is equivalent, therefore, to a resistance R and a reactance X in series as shown in Fig. 1A.

The equivalent reactance, X', necessitates a slight increase in the capacity of the variable condenser 2,'to tune the circuit to a given frequency. The equivalent resistance, R', adds ldirectly to the .resistance of the coil I, R, making the total circuitresistance equal to R-l-R, the condenser, 2, being assumed to have no resistance. Figure 3 represents graphically the ratio this being given by diagonal lines from the origin to various points on the semicircle. The horizontal axis represents the ratio proportional to frequencies.

It will be seen that if Ro and Co are so selected that at a certain frequency falls at the point a, (the resultant impedance ratio ZI E 'being vectorially shown by the dotted line from zero to point a), then with increasing frequency,

lthevalue of R is high and falls slowly while X creasing frequency causes both R' and X to decrease. If, as a third possibility,

falls at the point c at the middle of the semicircle, X is practically constant over a limited frequency range, and R' decreases as frequency increases.Vv

If the principle embodied in Figs. 1 and 3 is to be employed in band-pass tuned radio receivers employing a pair of coupled tuned circuits in each stage, or `in receivers employing a single tuned circuit in each stage, then a high ratio of maximum to minimum resistance is required and X' should fall in the region b, or between b and c, where although R. is much smaller than the resistance Ro the range of variation of' R' is very great, as will be readily perceived from the graphic representation in Fig. 3.

In Figure 4 there is graphically shown the total resistance of a tuned circuit, inco-rporating the impedance Ro, Cu in series, calculated for a coil whose inductance was 306 microhenries and whose resistance was found by actual measurement to be as shown by curve K in Figure 4. In curves A, B, and C, wRoCo was equal to 3 at a frequency of 600 kilocycles, and had proportional values at other frequencies. Rn was equal to 50, 100, and ohms respectively, in the three curves, and C0 to .0159, .00795, and .0053 microfarads, respectively. Curves D, E, and F were calculated for values of Rn of 25, 50 and 100 ohms, respectively, and of Cn of .0212, .01106 and .0053 microfarads, respectively.y At 600 kilocycles, wRnCo was equal to2. Curves G, H, I, and J were calculated for Values ofRo of 10, 20, 30, and 40 ohms, respectively, and of Co of .0265, .01325, .00885, and .006625,microfarads, respectively, and at 600 kilocycles, wRnCo was equal to 1.

Curve J represents a circuit in which the impedance Cu,Rn has been adjusted for higher total circuit resistance at 600 kilocycles than at 1500 kilocycles, while Curve I represents a circuit in which Co, Ru has been adjusted for equal total circuit resistances at the two limiting frequencies, 600 and 1500 kilocycles. All of the curves (except curve K) fall within the scope of the invention, since they diier only in the degree to which the resistance-correcting means is applied.

In Fig. 2, I have shown a modified embodiment of my invention, this form including the tuned circuit consisting of the inductance coil I and the variable capacity 2. An impedance net-work is inserted in series with the inductance I and capacity 2, the net-work consisting of an inductance Lu in series with a resistancefRnthe inductance and resistance being inishunt with a'series conf nection of Ya capacity Co and resistance Re. The impedance of the circuit Lo, RL, Co and Rc in Fig. 2 varies as shown by curves A .and B in Fig. 5.

This figure graphic-ally shows the aforementioned relationship'between impedance and frequency, the horizontal axis denoting frequencies and the vertical axis denoting impedance. With increasing frequency, R first increases andthen decreases. For the applications described in the foregoing paragraph the descending part of the curve would be used, the constants being so chosen that at the lowest frequencies to bereceived, R' would be, say, at the point'a, and at the highest frequency would be at the point b. By suitable choice of the circuit constants the resistance R can be made to have any two selected values of resistance at two selected frequencies.

In Figure 6, there is graphically shown the resistance and reactance at various frequencies, of a circuit consisting of Lo, RL, C0, and Re, arranged as in Figure 2. The constants of this circuit were as follows: Ln=7 x 10-6 henries, RL=40 ohms, Cn=0.66 X 10-8 farads, and Rc=0. As -an example of the interpretation of the curves in Figure 6, take the frequency 21rf=7 X106.'

At this frequency the circuit is exactly equivalent to a resistance of 8 ohms in series with a condenser having a reactance of 27 ohms, which is found by calculation to be a condenser of .529 x 10-8 farads capacity. R is therefore equal to 8, and X to 27. Values of R and X for other frequencies are given by the solid line curves in'Figure 6. The curves in dotted lines give the typical variation of R' and X' when RL and. Re are small.

In Figure 7, curve K shows the 'radio frequency resistance of the coil used for Figure 4. Curve M is a po-rtion of the R curve in Figure 6.' Curve O shows the total resistance, R-I-R', of the coil and the resistance-correcting circuit in series. Curve L gives the equivalent resistance R of a different resistance-correcting network, and curve N is the resistance of the complete circuit. Curve P shows the same thing, R-I-R, for still ano-ther resistance-correcting circuit, having the constants Lo=1.'75 x 10-6 henries, RL=10 ohms, 00:2.64 X 10-8 farads, and Rc=0.

In Fig. 8, I have shown an amplifying circuit of the band-pass type employing the embodiment shown in Fig; 1. The incoming'signal energy is impressed across the input terminals of a screen grid tube I', the output circuit of the tube being tuned by the variable capacity 2 and the inductance coil I, the coil I, being coupled as at M to the coil 6, in the input circuit of the succeeding screenA grid tube 9. The input circuit of the tube 9 is tuned by the variable capacity 5, and the amplified output of the tube 9 is impressed upon a succeeding stage and utilized'in any well-known fashion.

In the tuned output circuit of tube I and the tuned input circuit of the tube 9, there are connected the impedance circuits in series with the variable capacity and inductance coil in each tuning circuit. This impedance circuit as sho-wn arrangement willresultin a uniform amplification of all frequencies within the accepted band.

In Fig. 9 there is shown a tuned radio fre- 1 quency` amplifier circuit .which incorporates` :the

nlodicatioirtas` shownin 2;' Here; again, the .incoming signal energy lis impressed across the input circuitof ithe triode I0, the output circuit of the latter including an indue-tance coil' I If whichis inductively coupled, as at M,.to the coil I in the input circuit of the triode I2. The input circuit of the triode"I2'1is tuned by a variable capacity 2, there beingan impedance circuit in series with'the inductance coil I and the variablecapacity 2. The `impedance circuit, asshown in Fig. 2, consists of the-inductance Lu in series with the resistance RL, the latter two elements being in shunt withV the series connection between the capacity C and'resistance Re.

A series cf resonance curves of an amplifying circuit of the type shown in Fig. 9 are shown in Fig. 10.` Curve A shows the characteristic of the resonance curve` at 1400 kilocycles with, and

without, the extra impedance elements inserted in the tuned input circuit of the tube I 2,l ythe dotted curve representing'the characteristic with the impedance elements inserted, while the solid line curve represents the characteristic without the impedance elements included in the tuned input circuit.

Curve B represents the same state of `affairs at 1000 kilocycles, while curve C indicates a similar state of affairs at 600 kilocycles. It will be observed that as the frequencies impresseddecrease, the function of the impedance elements is more sharply defined. Itis believed that Fig. 10 illustrates very clearly the advantage to-be derived from'including impedance elements, according to my novel principles, in the tuned circuit of the amplifier, the dotted lines indicating that a substantially constant degree of sharpness of tuning is secured throughout a predetermined range of frequencies. This is shown by the fact that the three dotted curves are substantially the same in shape, andequally Wide across the base of the curves.

While I have indicated and described several systems for carrying my invention into effect,'it will be apparent to one skilled :in the art that my invention is b-y no means limited to the particular' organizations shown and described, but that many modifications' in the circuit arrangements, as well as in the apparatus employed, may rbe made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

l l; A constant resistance ltuned circuit ccnsisting ofthe combinationin vseriesfof an inducta'nce coil. a condenser anda parallel branch network, the network" having resistance and in" ductance in series in one branch, and resistance and capacity in series in the other branch.

25A multi-stage radio frequency amplifier having coupled tuned circuitsin each stage and means including resistance in at least vone of -said circuits forestablishing a constant resistance characteristic whereby with constant accepted band width the'amplification over the range of the accepted band is maintained uniform, at any midband frequency.

3. A multi-stagetuned radio frequency amplier comprising va chain of coupled circuits, aty least one of said circuits being tuned, an impedance network in said tuned circuit, said net work including a resistance in shunt with a capacity, the electrical constants of the net-work elements being so vproportioned that the magnitude of the effective resistance of said network is inversely proportional to the frequency towhich' the amplifier is tuned.

4. A multi-stage amplifier comprising a pair of coupled circuits at least one of said circuits being tuned, and means, including a plurality of reactances, in the tuned circuits for establishing a constant resistance characteristic whereby with constant accepted band Width the amplification over the range of the accepted band is maintained uniform, at any mid-band frequency.

5. A multi-stage radio frequency amplifier having coupled tuned circuits in each stage, and means, including a series impedance path, in one of the tuned circuits for establishing a constant resistance characteristic whereby with constant accepted band width the amplification over the range of the accepted band is maintained uniform, at any mid-band frequency.

6. In an amplifier, as defined in claim 5, said series impedance path containing resistance and capacity in parallel.

7. A multi-stage radio frequency amplifier, as defined in claim 2, said resistance being connected in parallel with a capacity.

8. A multi-stage radio frequency amplifier including at least one tuned circuit provided with a variable tuning condenser in each pair of coup-led input and output circuits, and an impedance path in said tuned circuit whose reactance and resistance is proportioned so that the circuit has a constant resistance characteristic over a preldetermined range of frequencies whereby the sharpness of tuning is substantially the same at all frequencies within the predetermined range.

9. A band pass amplifier including a chain of coupled circuits, at least two of which circuits 4are tuned, the tuned circuits possessing constant resistance over a predetermined range of frequencies whereby the band pass amplifier amplifles uniformly all frequencies Within the accepted band, each tuned circuit consisting of an inductance, a variable capacity, and a series impedance including at least a resistance and capacity in parallel.

10. A network, adapted to transmit a band of super-audible frequencies of substantially constant width over a tuning range with substantial uniformity. comprising a pair of coupled oscillatory circuits each including an inductance and capacity, means for vvarying the resonant frequency of said circuits to tune the network over :said range, and an impedance path in series with the inductance and capacity in each of said circuits, said path including reactance and resist ance of such magnitude that the over-all resistance of said networkdecreases as the network is tuned to the high frequency end of said range.

11. A super-audible frequency transmission network comprising an oscillation circuit consisting of an inductance. element and a shunt capacity element, said capacity element being adjustable in magnitude to vary the resonant frequency of the circuit over a range of frequencies, said circuit additionally including an impedance path in series with said elements Whose reactance and resistance is so proportioned that the resist-ance of said path decreases in a predetermined manner as the frequency of the circuit is adjusted to the high frequency end of said range.

12. A super-audible frequency transmission network comprising an oscillation circuit consisting of an inductance element and a shunt capacity element, said capacity element being adjustable in magnitude to vary the resonant frequency of the circuit over a range of frequencies, ,said circuit additionally including a series impedance path whose reactance and resistanceV is so proportioned that the total effective resistance of the circuit remains substantially constant over said range whereby a substantially constant degree of sharpness of tuning of said circuit is secured throughout said range.

13. In combination with a source of radio frequency energy and an electron discharge tube, a network comprising an oscillatory circuit connected to said source and a second oscillatory circuit connected between the input electrodes of said tube, said circuits being coupled to transfer energy from said source to said tube, means for tuning at least one of the circuits through a frequency range, and an impedance in said last circuit whose reactance andresistance is so proportioned that thetotal effective resistance of the circuit remains substantially constant over said range whereby a substantially constant degree of sharpness of tuning thereof is secured throughout said range.

14. In combination, apair of cascaded tubes, a coupling network between the tubes comprising at least two coupled oscillation' circuits each including an inductance coil and a condenser, Said both circuits being resonant to the same frequency, and a lumped impedance connected in series with the condenser of each circuit, said impedances comprising reactance and resistance so chosen that the effective resistance of the net- Work varies inversely with frequency.

15. In combination, a pair of cascaded tubes, a coupling network between'the tubes comprising at least two coupled oscillation circuits each including an inductance coil and a variable condenser, said both circuits being resonant to the same frequency, and a lumped impedance connected in series with the condenser and coil of each circuit, said impedances comprisingv reactance and resistance so chosen that the effective resistance of the network varies inversely with frequency.

16. In a tunable band pass network of the type comprising a pair of reactively coupled resonant oscillation circuits, said network being adapted to transmit a band of frequencies consisting of the modulation frequencies of a modulated carrier, means for tuning said circuits to common carrier frequencies of a desired carrier frequency range, an electron discharge tube load coupled to said network, an impedance device in at least one of said circuits whose resistance varies inversely with carrier frequency in such a manner that all the modulation frequencies of any carrier frequency selected from said range are transmitted through said network to said load with substantial uniformity.

17. In combination with an electron discharge tube, a resonant oscillation circuit, adapted to have modulated radio frequency currents of a desired frequency range impressed thereon, said circuit consisting of a coil, a variable tuning condenser yto tune it through said range, and an impedance device whose resistance automatically decreases as the circuit is tuned to the high frequency end of said range and currents of increasing frequency are impressed on said circuit.

18. In combination with an electron discharge tube, a resonant oscillation circuit, adapted to have modulated radio frequency currents of a desired frequency range impressed thereon, said circuit consisting of a coil, a variable tuning condenser to tune it through said range, and an impedance device whose resistance automatically decreases in such a manner as the circuit is tuned to the high frequency end of said range and currents of increasing frequency are impressed on said circuit that said circuit is provided with a constant selectivity characteristic.

19. A high frequency electrical coupling system including at least one resonant circuit tunable throughout a frequency range, and xed impedance means included in said resonant circuit producing eiects so variable With frequency as to provide approximately the same resonance band widths for said coupling system at the upper and lower frequency limits of said tunable range.

20. A high frequency electrical coupling system including at least one resonant circuit tunable throughout a frequency range, and fixed impedance means included in said resonant circuit producing effects so variable with frequency as to minimize changes in selectivity of said coupling system as the tuning is adjusted throughout said frequency range.

21. A high frequency electrical coupling system including at least one resonant circuit tunable throughout a frequency range, and energy dissipating impedance means decreasing in disspative eifect with increase in frequency included in said resonant circuit for reducing variations in selectivity of said coupling system throughout said tunable range.

22. A high frequency electrical coupling system including at least one resonant circuit tunable throughout a frequency range, and fixed impedance means comprising resistance Shunted by capacity serially interposed in said resonant circuit for reducing variations in selectivity of said coupling system as the tuning is adjusted throughout said frequency range.

23. A high frequency electrical coupling system comprising a resonant circuit, means for variably tuning said system over a range of frequency and means for causing the power factor of said system to increase when the frequency to which said system is tuned decreases.

24. A high frequency electrical coupling system comprising a resonant circuit, means for variably tuning said system over a range in frequency and means for introducing resistance into said circuit which effectively increases in magnitude with decreasing frequency.

25. A high frequency electrical coupling system comprising a tunable circuit, means for varably tuning said circuit over a range in frequency, means introducing resistance into said circuit and means automatically varying the effective magnitude of said resistance inversely with respect to the tuning frequency, whereby a substantially vconstant degree of selectivity is maintained over said frequency range.

26. A high frequency electrical coupling systern tunable throughout a range in frequency, adapted to interconnect successive portions of a multi-stage amplifier, said system comprising a primary circuit and a tunable resonant secondary circuit inductively coupled to said primary circuit and means associated with said secondary circuit for introducing losses therein which vary as said circuit is tuned throughout said range to maintain substantially constant the selectivity of said system throughout said tunable frequency range.

RENE A. BRADETN. 

